1. Field of the Invention
This invention relates to a method for the measurement of the velocity of the surface acoustic wave generated on a test piece when an acoustic wave is projected thereon.
2. Description of the Prior Art
A method of inspecting a test piece for inner defects by injecting an acoustic wave into the test piece has been known to the art.
The acoustic lens which is employed in acoustic microscopy, for example, has been known to possess an ability to excite and detect leaky surface acoustic waves. Recently, a method of making use of this ability of the acoustic lens in effecting the measurement of the velocity of the surface acoustic wave which is generated on a test piece when the test piece is irradiated with an acoustic wave has been reported. This method is based on the principle that when the acoustic lens is moved in a perpendicular direction relative to the surface of the test piece and the amplitude V of the reflected wave is measured as the function of the distance z between the surface of the test piece and the focal point of the lens, the velocity of the surface acoustic wave of the test piece can be determined from the minimal cyclic interval .DELTA.z appearing in the V(z) curve obtained. Since this method is capable of accurately measuring the elastic property of a solid surface in a microscopic area on the order of 1 to 100 microns, it is attracting mounting interest as a new nondestructive testing method advantageously applicable to tools, mechanical parts, and electronic components which have undergone coating with PVD, CVD, etc. surface hardening, carburizing and other surface treatments.
The measurement of the velocity of the surface acoustic wave by the method described above, however, inevitably entails an operation of mechanically moving the acoustic lens in a perpendicular direction relative to the surface of the test piece with the aid of a micrometer, for example. The measurement of the velocity at one point on the test piece requires at least several seconds. Besides, the mechanical accuracy of this measurement poses a problem. When the measurement of velocity is made at a multiplicity of points on the surface of a test piece to find the distribution of velocity on the surface or when real-time measurement is performed on a test piece whose surface condition varies from moment to moment, this method poses a problem with respect to the speed with which the measurement operation can be carried out.